Don't do a full length weld, you will end up warping the axle most likely.  Tack weld it really good on all the major corners, then stitch weld on opposite alternating sides and location to prevent overheating and warpage.  After the stitch welds have sufficently cooled you can go back through and re-stitch the unwelded portions for a fully sealed truss.  I personally left mine stitch welded.